Board Thread:Pokémon Help/@comment-35078634-20180402115741/@comment-32358261-20180403035125

No the happiness would be the same. Even if you kept them in the on for a long time. Happiness only resets when it’s given to a new trainer. It would go back to max if returned to original trainer. At the battle Collosuem, even if pokemon faints or uses an item (like a berry or focus sash), they remain intact after that battle ends. Happiness can not be gained there either.